Data Communications Networking Devices - 4th Ed.pdf

352 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTAcoustic couplers normally use two distinct frequencies to transmit information,while two other frequencies are employed for data reception. A frequency fromeach pair is used to create a mark tone which represents an encoded binary one fromthe digital data stream, while another pair of frequencies generates a space tonewhich represents a binary zero. This utilization of two pairs of frequencies permitsfull-duplex transmission to occur over the two-wire switched telephone network.Since acoustic couplers enable any conventional telephone to be used for datatransmission purposes, the coupler does not have to be physically wired to the line.This permits considerable ¯exibility in selecting a transmission location, which caninclude pay telephones in airports and hard-wired telephones in hotel rooms.Acoustic couplers are manufactured both as separate units and as built-in units todata terminals, as shown in Figure 4.1. Due to the signi®cant replacement ofportable terminals by laptop computers with built-in modems with modular jacks,the market for both portable terminals and acoustic couplers has greatly diminished.However, if you read some mobile computing trade magazines you will still noteadvertisements for battery-powered acoustic couplers as they permit the transmissionmobility many travelers require.Figure 4.1 Varying coupler connections: a) terminal with built-in coupler; b) terminalconnected to couplerUS and European compatibilitySince acoustic couplers are normally employed to permit portable data processingdevices to communicate with data-processing facilities, and since a large portion oflow-speed modems at such facilities in the United States were originally furnishedby AT&T and its operating companies prior to its break-up into independentorganizations, most manufacturers of acoustic couplers designed them to be compatiblewith low-speed `Bell System' modems. Here the term `Bell System' refersto the operating characteristics of modems that were manufactured by WesternElectric for use by AT&T operating companies prior to those operating companiesbecoming independent organizations.In Europe, most acoustic couplers are designed to be compatible with ITUrecommendations that govern the operation of low-speed modems. To understandthe differences between low speed Bell System and ITU modems, we will examineacoustic couplers that operate at data rates between 0 and 450 bps. In the UnitedStates, such couplers are compatible with Bell System 103 and 113 type modemswhile in Europe such couplers are compatible with the ITU V.21 recommendation.Table 4.1 lists the operating frequencies of acoustic couplers designed to operatewith Bell System 103/113 type modems and modems that follow the ITU V.21recommendation.